1. Field of the Invention
The invention relates to the construction of whip antennas from discrete blades.
2. Description of the Related Art
The physical size of an antenna largely depends upon the purpose for which it is to be employed. For example, an antenna for receiving a particular frequency range must have an electrical length capable of resonating within that range to achieve optimum reception. Generally, lower frequencies require longer lengths because the wavelengths at lower frequencies are longer, but limitations in use often demand design modifications to achieve appropriate electrical length in a smaller space. A whip antenna is an example of a widely used antenna. It has a thin flexible core and may be attached to a vehicle; the name is derived from the whip-like motion of the antenna when disturbed. It is not uncommon for such antennas to be 10 feet or more in length.
FIGS. 1-4 illustrate prior art constructions of whip antenna assemblies. FIGS. 1 and 2 show a well-known prior art construction of a whip antenna assembly 10 comprising a number of pairs of leaves 12 nested together in a stack 13 and secured to each other by rivets 14. Each leaf or blade is curved in cross section much like a tape measure, and each pair has a different length. The longest pair 12A is stacked in the middle, and sandwiched between the two leaves of the next longest pair, which are in turn sandwiched between the two leaves of the next longest pair and so on. The distal end of each leaf has an elongated slot, and each interior leaf of the stack has an elongated slot in registry with the distal elongated slots so that a rivet can extend through the elongated slots in the stack. The rivets are not tight, thus enabling the stack to bend as shown in FIG. 3.
The proximate end 16 of the stack 13 is secured to a coupler 18 which comprises a connector 20 to enable an electrical connection to a receiver or transmitter. A hook and loop strap 22 is attached to the stack near the proximate end 16 so that when the distal end 24 of the antenna is bent against itself it can be secured by the hook and loop strap to make it easier to transport the antenna.
A hole is often provided near the distal end of the longest pair of leaves so that a line can be attached to the distal end of the antenna. In the field, such a line enables the antenna to be pulled up into a tree or other object for mounting at a higher elevation to improve transmission and reception of signals via the antenna.
One of the problems with the prior art design of FIGS. 1 and 2 is that dirt and other contaminants become lodged between the leaves of the stack. Such contaminants interfere with the bending of the stack for transport and eventually degrade the durability of the antenna.
A solution to the problem was found in the prior art antenna assembly 30 of FIG. 4. Here, two stacks 32, 34 of leaves or blades, preferably arcuate in cross section, are secured at a proximal end 36 thereof to a fusiform pin 38 extending from an electrically conductive coupler 40 that preferably matches the curvature of the blades. The proximal end of each stack 32, 34 is fixedly secured to the pin 38 on opposite sides thereof by rivets 42, 44, and each stack comprises a nested number of blades of different lengths with the longer blades on the outside of the stack and shorter blades on the inside of the stack. The remaining portions of the blades are free to slidably move relative to each other as the stacks 32, 34 are bent. The entire assembly is covered with a heat shrink wrap (not shown) that protects the stacks from the introduction of contaminants and water, yet enables the leaves or blades to move relative to each other so that the whip antenna can be folded for ease of transport. A cap (not shown) is typically glued or welded to the heat shrink wrap at the distal end.
One of the problems with the prior art design of FIG. 4 is that there is no easy and effective way to secure a line to the distal end of the antenna. Adding a hole to the distal end compromises the integrity of the heat shrink wrap. As well, the bond between the cap and the heat shrink wrap may not be strong enough to enable a line to be secured to the cap and pull the antenna and associated structures by way of the cap. Either the cap will become dislodged or if the bond is strong enough, the heat shrink wrap will inelastically deform when the line pulls the structure against gravity.